[swift-evolution] [Pitch] Retiring `where` from for-in loops

[Xiaodi Wu]

On Mon, Jun 13, 2016 at 8:16 AM, Brandon Knope <bknope at me.com> wrote:

> Are you really surprised that some people don't want this taken away?
>

Nope, that's to be expected.


> The burden should be on those that want it taken out of the language and
> not those that want it kept. After all something is being removed and it
> should be a delicate process.
>

Agreed. We who think it's better to take this syntax out have advanced an
argument with several prongs. Namely, that the `where` clause serves no
independent purpose; that a more general solution has already been added to
the language (as well as another in the stdlib); that the `where` clause is
not necessary for progressive disclosure to new users before they're ready
for the general solution; that it is, at present, rarely used in practice;
that it has no analog in other commonly used general purpose languages in
the C family; that it is the remnant of a direction in which the core team
later decided not to pursue; and that, given its lack of utility, lack of
use, and vestigial state, being the cause of confusion even among a small
number of users (if their number be small) is grounds to conclude that it
is harmful to the language and therefore ought to be removed.


> Don't be surprised when the defenders say it is more readable to them.
> That is a *sound* argument in my opinion.
>

IMO, it cannot stand on its own as a complete argument for saving a feature
in the face of the arguments we've advanced. Couldn't you say the same for
`++` or `for;;` loops? I'd say our case is at least as strong as that for
`for;;` loops. By comparison, if I recall, the `for;;` loop was argued to
be ill-fitting the rest of the language and lacking in usage, but it
certainly had utility independent of `for...in` loops and was well
precedented in C languages.


>
> Brandon
>
> Sent from my iPad
>
> On Jun 13, 2016, at 8:33 AM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> This is not a sound argument. If your filtering can be expressed as a
> where clause, then you would only have to read one line into the loop to
> see it in the form of a guard clause.
>
> Moreover, if what you're arguing is that you shouldn't ever have to *read*
> inside the loop to know if a sequence is filtered, how do you propose that
> we do that? Remove the continue keyword?
>
> On Mon, Jun 13, 2016 at 6:16 AM Jean-Daniel Dupas via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> -1 for the removal.
>>
>> When I read code, I find it far more visible that a loop is over a filter
>> list when the filter clause is on the same line, than when the filter
>> clause is inside the loop.
>>
>> Having to read the full content of the loop to determine if the list is
>> filtered or not is not an improvement IMHO.
>>
>> Moreover, I find it far cleaner to use the where clause that having to
>> remember than I have to use the lazy accessor to avoid a performance hit.
>>
>> Le 13 juin 2016 à 06:39, Charlie Monroe via swift-evolution <
>> swift-evolution at swift.org> a écrit :
>>
>> And to follow-up to myself once again, I went to my "Cool 3rd Party Swift
>> Repos" folder and did the same search. Among the 15 repos in that folder, a
>> joint search returned about 650 hits on for-in (again with some false
>> positives) and not a single for-in-while use.
>>
>> -- E
>>
>>
>> Not to undermine this fact, but I believe the fact that `where` can be
>> used in a for loop is not widely known. I didn't know about it until about
>> a month ago (haven't really read much docs, but most people don't either).
>>
>> But after I found out about it, I started using it and it IMHO improved
>> readability of my code. Not by much, but it's the little things that make
>> you smile, right?
>>
>> Many people here argument that `where` is a Swift speciality and needs to
>> be learned by the developer - the alternative is to teach the person what's
>> the proper alternative - that using .filter can have performance impact and
>> that the *correct* way is to use guard within the for loop. And that's IMHO
>> much worse than teaching a person about using `where` within a for loop.
>>
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